Articles having surfaces capable of spreading water are desirable for a variety of uses. For example, transparent plastics used in misty or humid environments, such as windows of greenhouses, should avoid the formation of light-reflecting small water droplets which reduce light transmission. A water-spreading surface on these materials helps to maintain their transparency and minimizes undesirable streaking. Such water-spreading coatings, also referred to as anti-fogging coatings, are described e.g. in WO 2009/140482 (Jing et al.) and WO 2010/017069 (Jing et al.). The anti-fogging coatings disclosed in the above references in the context of transparent substrates, are claimed to resist the formation of small, condensed water droplets which would otherwise significantly reduce the transparency of the substrate. Other anti-fogging coating compositions are disclosed in WO 2009/085680 (Chen et al.) and WO 2011/002838 (Jing et al.), wherein the coating compositions comprising surface functionalized silica nanoparticles are described to be particularly useful on personal protection equipment such as face masks, shields and protective glasses.
Water-spreading characteristics are also desirable on articles used in outdoor applications, such as traffic signs that employ retro-reflective sheeting. Retro-reflective sheeting has the ability to return substantial quantities of incident light back towards the light source. Such light transmission into and back from a retro-reflective sheeting is impaired by clinging raindrops and in particular dew formation.
A prominent form of precipitation that affects light transmission or light reflection is dew formation. Dew can be particularly problematic because it occurs predominantly at nighttime when the retro-reflective sheeting is operative. When present on a traffic sign in the form of large beaded water droplets, dew can disrupt the path of incident and retro-reflective light. This can make information on the sign much more difficult for passing motorists to read. In some cases, accumulation of large water droplets on the surface of the retro-reflective traffic sign may result in complete loss of reflectivity or to a so-called “blackout” according to which the traffic sign becomes completely ineffective in displaying useful, if not vital, information to motorists or observers. In contrast, when the dew is spread out smoothly as a transparent layer over the surface of the retro-reflective traffic sign, the information on the sign is much easier to read because the resulting thin smooth layer of water does not significantly misdirect the path of incident and retro-reflective light to as great an extent.
The coatings disclosed in the art are not always satisfactory when it comes to prevent dew formation, especially on the surface of retro-reflective articles used in outdoor applications. Preventing or reducing dew formation, in particular in an outdoor environment, is more of a stringent requirement than providing anti-fogging, especially in terms of durability, UV-stability and/or abrasion resistance provided by the corresponding coating.
WO 2005/103172 (Iyer et al.) discloses a so-called dew-resistant coating, wherein the described dew-resistant coating comprises elongate silica particles having an aspect ratio greater than 1. Other alternative ways of preventing dew formation on substrates are described in the background section of this patent application.
Without contesting the technical advantages associated with the anti-fogging and dew resistant coatings disclosed in the art, there is still a need for a method of retarding dew formation on the surface of an article comprising a retro-reflective support, and for coated articles and coatings having improved performance with regard to dew formation retarding, durability, UV-stability and/or abrasion resistance, in particular when applied on the surface of retro-reflective articles used in outdoor applications.
Other advantages of the coatings, coated articles and methods of the disclosure will be apparent from the following description.